Gesso can be thought of as the bridge between your support (canvas, board or whatever you paint on) and the paint.
Pastes can be used to achieve a variety of effects and expressions.
Gel Mediums offer many ways to build texture. They can be used to create glazes, extend paint and change finishes. Gels are essentially colorless paint.
Glazing Mediums are used to create glazes, soft transitions and color blending in paintings.